1. Field of the Invention
This invention relates to a material for a diamond sintered compact tool (which will sometimes be referred to as xe2x80x9cdiamond sintered compactxe2x80x9d simply), which has a high strength and is available as a material for a cutting tool, and a process for the production of the same.
2. Description of the Prior Art
At the present time, diamond sintered compacts have been produced for applying to industrial uses such as cutting tools, dies, bits, etc., used for cutting nonferrous metals, plastics, ceramics, etc., and used as a dresser, drill bit, wire drawing die, etc. In particular, a diamond sintered compact sintered at an ultra-high pressure and high temperature using a binder material containing an iron group metal such as cobalt, etc. as a predominant component has widely been used for cutting nonferrous metals, plastics, ceramics, etc., for dressers, drill bits and wire drawing dies.
Among these tools, in particular, a diamond sinterd compact tool used for a cutting tool is generally composed of a tool material comprising a diamond sintered compact layer having a thickness of about 0.5 to 1.5 mm and a WC-Co type cemented carbide substrate bonded therewith during a step where diamond grains are sintered with each other, the tool material being cut in a predetermined shape to give a cutting edge of a tool and then brazed to a base of a cemented carbide.
In processes for the production of diamond sintered compacts according to the prior art, for example, there is employed a method comprising arranging a metallic sheet of an iron group metal such as cobalt to be in contact with diamond powder as shown in FIG. 2, or mixing a metallic powder of an iron group metal such as cobalt with diamond powder, then allowing the iron group metal to be melted and impregnated in the diamond powder and simultaneously sintering the diamond powder at an ultra-high pressure and high temperature (Cf. JP-A-58-199777). However, the diamond sintered compact obtained by this method is not satisfied with respect to strength, because thermal stress remains in the sintered compact due to difference in thermal expansion between the diamond sintered compact layer and cemented carbide substrate and the strength is lowered by the residual stress during sintering.
The above described diamond sintered compact has excellent properties as a cutting tool for nonferrous metals and has widely been used, but on the other hand, has a problem that the strength is low due to the residual stress during sintering in spite of its excellent wear resistance. That is, in the material for a diamond sintered compact tool of the prior art, thermal stress remains in the sintered compact due to difference in thermal expansion between the diamond sintered compact layer and cemented carbide substrate and this residual stress has a bad influence upon the strength of the diamond sintered compact, so it can readily be assumed that the residual stress should be reduced in order to improve the strength. Since the thermal stress is due to difference in thermal expansion between the diamond sintered compact layer and cemented carbide substrate during sintering and complete removal of the thermal stress is difficult, however, no effective countermeasure to reduce the internal stress has been proposed. For example, a method of controlling the grain size of diamond grains or changing the kind of a binder to be employed has been proposed as a means for improving the strength of the diamond sintered compact, but the effects of such a method are limited.
It is an object of the present invention to provide a material for a high strength diamond sintered compact tool, whereby the above described problems of the prior art can be solved.
It is another object of the present invention to provide a process for the production of a high strength diamond sintered compact by sintering diamond grains with a binder, i.e. Co diffusing from a cemented carbide substrate.
These objects can be attained by a diamond sintered compact comprising a WC-Co type cemented carbide substrate having slight undulation and a diamond sintered compact bonded to one surface of the substrate by sintering during a step of sintering at an ultra-high pressure and high temperature, which has a plate thickness of 0.5 mm to 5 mm and an outer diameter of at least 20 mm and whose diamond sintered compact layer has at least 50% of a thickness area within a range of 0.05 mm to 0.4 mm and contains Co diffusing from the cemented carbide substrate. The WC-Co type cemented carbide used herein generally contains 4 to 20 weight % Co.